among the “multitudinous” stellar hosts there are probably many stars
having life-bearing planets revolving round them.
THE SUN—WHAT WE LEARN FROM IT.—RICHARD A. PROCTOR
The Sun, the central and ruling body of the planetary system, and
the source of light and heat to our earth and all the members of
that system, is a globe about 852,900 miles in diameter. So far as
observation extends, his figure is perfectly spherical, no difference
having been observed between his polar and spherical diameters. It
has been well remarked, indeed, by Sir G. Airy, that if any observer
could by ordinary modes of measurement satisfy himself that a real
difference existed between the diameters, that observer would have
proved the inexactness of his own work; for the absence of any
measurable compression comes out as the result of comparisons between
thousands of observations of the sun’s limbs made at Greenwich
and other leading observatories. The volume of the sun exceeds
the earth’s 1,252,700 times. His mean density is almost exactly
one-fourth of the earth’s, and his mass exceeds hers about 316,000
times. Gravity at the surface of the sun exceeds terrestrial gravity
about 27.1 times, so that a body dropped from rest near the sun’s
surface would fall through 436 feet in the first second, and have
acquired a velocity of 872 feet per second.
Let the reader consider a terrestrial globe three inches in diameter,
and search out on that globe the tiny triangular speck which
represents Great Britain. Then let him endeavor to picture the town
in which he lives as represented by the minutest pin-mark that could
possibly be made upon this speck. He will then have formed some
conception, though but an inadequate one, of the enormous dimensions
of the earth’s globe, compared with the scene in which his daily life
is cast. Now, on the same scale, the sun would be represented by a
globe about twice the height of an ordinary sitting-room. A room
about twenty-six feet in length, and height, and breadth, would be
required to contain the representation of the sun’s globe on this
scale, while the globe representing the earth could be placed in a
moderately large goblet.
Such is the body which sways the motions of the Solar System. The
largest of his family, the giant Jupiter, though of dimensions which
dwarf those of the earth or Venus almost to nothingness, would yet
only be represented by a thirty-two inch globe, on the scale which
gives to the sun the enormous volume I have spoken of. Saturn would
have a diameter of about twenty-eight inches, his ring measuring
about five feet in its extreme span. Uranus and Neptune would be
little more than a foot in diameter, and all the minor planets would
be less than the three-inch earth. It will thus be seen that the sun
is a worthy centre of the great scheme he sways, even when we merely
regard his dimensions.
[Illustration: Fig. 29.—Sun Spot seen in 1870]
The sun outweighs fully seven hundred and forty times the combined
mass of all the planets which circle around him, so that, when we
regard the energy of his attraction, we still find him a worthy ruler
of the planetary scheme.
Viewed with the naked eye, the sun appears only as a luminous mass of
intense and uniform brightness; but when examined with the telescope,
his surface is frequently observed to be mottled over with a number
of dark spots, of irregular and ill-defined forms, constantly
varying in appearance, situation, and magnitude. These spots are
occasionally of immense size, so as to be visible even without the
aid of the telescope; and their number is frequently so great that
they occupy a considerable portion of the sun’s surface. Sir W.
Herschel observed one in 1779 the diameter of which exceeded 50,000
miles, more than six times the diameter of the earth; and Scheiner
affirms that he has seen no less than fifty on the sun’s disk at
once. Most of them have a deep black nucleus, surrounded by a fainter
shade, or _umbra_, of which the inner part, nearest to the nucleus,
is brighter than the exterior portion. The boundary between the
nucleus and umbra is in general tolerably well defined; and beyond
the umbra a stripe of light appears more vivid than the rest of the
sun.
[Illustration: Fig. 30.—Phase of Spot]
The discovery of the sun’s spots has been attributed to Fabricius,
Galileo, and Scheiner, and has been claimed for the English
astronomer Harriot. Amid these conflicting pretensions it is perhaps
impossible to arrive at the truth; but the matter is of little
importance; the discovery is one which followed inevitably that of
the telescope, and an accidental priority of observation can hardly
be considered as establishing any claim to merit.
The study of solar physics may be said to have commenced with the
discovery of the sun spots, about two hundred and sixty years ago.
These spots were presently found to traverse the solar disk in such
a way as to indicate that the sun turns upon an axis once in about
twenty-six days. Nor will this rotation appear slow, when we remember
that it implies a motion of the equatorial parts of the sun’s surface
at a rate exceeding some seventy times the motion of our swiftest
express train.
Next came the discovery that the solar spots are not surface stains,
but deep cavities in the solar substance. The changes of appearance
presented by the spots as they traverse the solar disk led Dr. Wilson
to form this theory so far back as 1779; but, strangely enough, it
is only in comparatively recent times that the hypothesis has been
finally established, since even within the last ten years a theory
was put forward which accounted satisfactorily for most of the
changes of appearance observed in the spots, by supposing them to be
due to solar clouds hanging suspended at a considerable elevation
above the true photosphere.
Sir William Herschel, reasoning from terrestrial analogies, was led
to look on the spot-cavities as apertures through a double layer
of clouds. He argued that, were the solar photosphere of any other
nature, it would be past comprehension that vast openings should
form in it, to remain open for months before they close up again.
Whether we consider the enormous rapidity with which the spots form
and with which their figure changes, or the length of time that many
of them remain visible, we find ourselves alike perplexed, unless we
assume that the solar photosphere resembles a bed of clouds. Through
a stratum of terrestrial clouds openings may be formed by atmospheric
disturbances, but while undisturbed the clouds will retain any form
once impressed upon them, for a length of time corresponding to the
weeks and months during which the solar spots endure.
And because the solar spots present two distinct varieties of light,
the faint penumbra and the dark umbra or nucleus, Herschel saw the
necessity of assuming that there are two beds of clouds, the outer
self-luminous and constituting the true solar photosphere, the inner
reflecting the light received from the outer layer, and so shielding
the real surface of the sun from the intense light and heat which it
would otherwise receive.
But while recent discoveries have confirmed Sir William Herschel’s
theory about the solar cloud-envelopes, they have by no means given
countenance to his view that the body of the sun may possibly
be cool. The darkness of the nucleus of a spot is found, on the
contrary, to give proof that in that neighborhood the sun is hotter,
because it parts less readily with its heat. We shall see presently
how this is. Meantime let it be noticed, in passing, that a close
scrutiny of large solar spots has revealed the existence of an
intensely black spot in the midst of the umbra. This black spot must
be regarded as the true nucleus.
The circumstance that the spots appear only on two bands of the sun’s
globe, corresponding to the sub-tropical zones on our own earth, led
the younger Herschel to conclusions as important as those which his
father had formed. He reasoned, like his father, from terrestrial
analogies. On our own earth the sub-tropical zones are the regions
where the great cyclonic storms have their birth, and rage with their
chief fury. Here, therefore, we have the analogue of the solar spots,
if only we can show reason for believing that any causes resembling
those which generate the terrestrial cyclone operate upon those
regions of the sun where the solar spots make their appearance.
We know that the cyclone is due to the excess of heat at the earth’s
equator. It is true that this excess of heat is always in operation,
whereas cyclones are not perpetually raging in sub-tropical climates.
Ordinarily, therefore, the excess of heat does not cause tornadoes.
Certain aerial currents are generated whose uniform motion suffices,
as a rule, to adjust the conditions which the excess of heat at the
equator would otherwise tend to disturb. But when through any cause
the uniform action of the aerial currents is either interfered with
or is insufficient to maintain equilibrium, then cyclonic or whirling
motions are generated in the disturbed atmosphere, and propagated
over a wide area of the earth’s surface.
Now we recognize the reason of the excess of heat at the earth’s
equator in the fact that the sun shines more directly upon that part
of the earth than on the zones which lie in higher latitudes. Can
we find any reason for suspecting that the sun, which is not heated
from without as the earth is, should exhibit a similar peculiarity?
Sir John Herschel considers that we can. If the sun has an atmosphere
extending to a considerable distance from his surface, then there
can be little doubt that, owing to his rotation upon his axis, this
atmosphere would assume the figure of an oblate spheroid, and would
be deepest over the solar equator. Here, then, more of the sun’s
heat would be retained than at the poles, where the atmosphere is
shallowest. Thus, that excess of heat at the solar equator which
is necessary to complete the analogy between the sun spots and
terrestrial cyclones seems satisfactorily established.
It must be remarked, however, that this reasoning, so far as the
excess of heat at the sun’s equator is concerned, only removes
the difficulty a step. If there were indeed an increased depth of
atmosphere over the sun’s equator sufficing to retain the requisite
excess of heat, then the amount of heat we receive from the sun’s
equatorial regions ought to be appreciably less than the amount
emitted from the remaining portions of the solar surface. This is
not found to be the case, so that either there is no such excess of
absorption, or else the solar equator gives out more heat, in other
words, is essentially hotter, than the rest of the sun. But this is
just the peculiarity of which we want the interpretation.
It may be taken for granted, however, that there is an analogy
between the sun spots and terrestrial cyclonic storms, though as yet
we are not very well able to understand its nature.
Then next we come to one of the most interesting discoveries ever
made respecting the sun—the discovery that the spots increase and
diminish in frequency in a periodic manner. We owe this discovery to
the laborious and systematic observations made by Herr Schwabe of
Dessau.
Schwabe found, in the course of about ten and a half years, the solar
spots pass through a complete cycle of changes. They become gradually
more and more numerous up to a certain maximum, and then as gradually
diminish. At length the sun’s face becomes not only clear of spots,
but a certain well-marked darkening around the border of his disk
disappears altogether for a brief season. At this time the sun
presents a perfectly uniform disk. Then gradually the spots return,
become more and more numerous, and so the cycle of changes is run
through again.
The astronomers who have watched the sun from the Kew Observatory
have found that the process of change by which the spots sweep
in a sort of “wave of increase” over the solar disk is marked by
several minor variations. As the surface of a great sea wave will be
traversed by small ripples, so the gradual increase and diminution in
the number of the solar spots are characterized by minor gradations
of change, which are sufficiently well marked to be distinctly
cognizable.
[Illustration: Fig. 31.—Ptolemaic System]
There seems every reason for believing that the periodic changes
thus noticed are due to the influence of the planets upon the solar
photosphere, though in what way that influence is exerted is not at
present perfectly clear. Some have thought that the mere attraction
of the planets tends to produce tides of some sort in the solar
envelopes. Then, since the height of a tide so produced varies as
the cube or third power of the distance, it has been thought that
a planet when in perihelion would generate a much larger solar tide
than when in aphelion. So that, as Jupiter has a period nearly equal
to the sun-spot period, it has been supposed that the attractions
of this planet are sufficient to account for the great spot period.
Venus, Mercury, the Earth, and Saturn have, in a similar manner,
been rendered accountable for the shorter and less distinctly marked
periods.
Without denying that the planets may be, and probably are, the
bodies to whose influence the solar-spot periods are to be ascribed,
I yet venture to express very strong doubts whether the attraction
of Jupiter is so much greater in perihelion than in aphelion as to
account for the fact that, whereas at one season the face of the sun
shows many spots, at another it is wholly free from them.[23]
However, we are not at present concerned so much with the explanation
of facts as with the facts themselves. We have to consider rather
what the sun is and what he does for the Solar System than why these
things are so.
Let us note, before passing to other circumstances of interest
connected with the sun, that the variable condition of his
photosphere must cause him to change in brilliancy as seen from
vast distances. If Herr Schwabe, for instance, instead of observing
the sun’s spots from his watch-tower at Dessau, could have removed
himself to a distance so enormous that the sun’s disk would have
been reduced, even in the most powerful telescope, to a mere point
of light, there can be no doubt that the only effect which he would
have been able to perceive would have been a gradual increase and
diminution of brightness, having a period of about ten and a half
years.
Our sun, therefore, viewed from the neighborhood of any of the stars,
whence undoubtedly he would simply appear as one among many fixed
stars, would be a “variable,” having a period of ten and a half
years. And further, if an observer, viewing the sun from so enormous
a distance, had the means of very accurately measuring its light, he
would undoubtedly discover that, while the chief variation of the
sun takes place in a period of ten and a half years, its light is
subjected to minor variations having shorter periods.
The discovery that the periodic changes of the sun’s appearance are
associated with the periodic changes in the character of the earth’s
magnetism is the next that we have to consider.
It had long been noticed that, during the course of a single day, the
magnetic needle exhibits a minute change of direction, taking place
in an oscillatory manner. And, when the character of this vibration
came to be carefully examined, it was found to correspond to a sort
of effort on the needle’s part to turn toward the sun. For example,
when the sun is on the magnetic meridian, the needle has its mean
position. This happens twice in a day, once when the sun is above
the horizon and once when he is below it. Again, when the sun is
midway between these two positions—which also happens twice in the
day—the needle has its mean position, because the northern and the
southern ends make equal efforts (so to speak) to direct themselves
toward the sun. Four times in the day, then, the needle has its mean
position, or is directed toward the magnetic meridian. But, when the
sun is not in one of the four positions considered, that end of the
needle which is nearest to him is slightly turned away from its mean
position toward him. The change of position is very minute, and only
the exact modes of observation made use of in the present age would
have sufficed to reveal it. There it is, however, and this minute
and seemingly unimportant peculiarity has been found to be full of
meaning.
The minute vibrations of the magnetic needle, thus carefully
watched—day after day, month after month, year after year—were found
to exhibit a yet more minute oscillatory change. They waxed and waned
within narrow limits of variation, but yet in a manner there was
no mistaking. The period of this oscillatory change was not to be
determined, however, by the observations of a few years. Between the
time when the diurnal vibration was least until it had reached its
greatest extent, and thence returned to its first value, no less than
ten and a half years elapsed, and a much longer time passed before
the periodic character of the change was satisfactorily determined.
The reader will at once see what these observations tend to. The
sun spots vary in frequency within a period of ten and a half years,
and the magnetic diurnal vibrations vary within a period of the same
duration. It might seem fanciful to associate the two periodic series
of changes together, and doubtless when the idea first occurred to
Lamont, it was not with any great expectation of finding it confirmed
that he examined the evidence bearing on the point. Judging from
known facts, we may see reasons for such an expectation in the
correspondence of the needle’s diurnal vibration with the sun’s
apparent motion, and the law which has been found to associate the
annual variations of the magnet’s power with the sun’s distance. But
undoubtedly when the idea occurred to Lamont it was an exceedingly
bold one, and the ridicule with which the first announcement of the
supposed law was received, even in scientific circles, suffices to
show how unexpected that relation was which is now so thoroughly
established. For a careful comparison between the two periods has
demonstrated that they agree most perfectly, not merely in length,
but maximum for maximum, and minimum for minimum. When the sun spots
are most numerous, then the daily vibration of the magnet is most
extensive, while, when the sun’s face is clear of spots, the needle
vibrates over its smallest diurnal arc.
Then the intensity of the magnetic action has been found to depend
upon solar influences. The vibrations by which the needle indicates
the progress of those strange disturbances of the terrestrial
magnetism which are known as magnetic storms have been found not
merely to be most frequent when the sun’s face is most spotted, but
to occur simultaneously with the appearance of signs of disturbance
in the solar photosphere. For instance, during the autumn of 1859,
the eminent solar observer, Carrington, noticed the apparition of
a bright spot upon the sun’s surface. The light of this spot was
so intense that he imagined the dark glass which protected his eye
had been broken. By a fortunate coincidence, another observer, Mr.
Hodgson, happened to be watching the sun at the same instant, and
witnessed the same remarkable appearance. Now it was found that
the self-registering magnetic instruments of the Kew Observatory
had been sharply disturbed at the instant when the bright spot was
seen. And afterward it was learned that the phenomena which indicate
the progress of a magnetic storm had been observed in many places.
Telegraphic communication was interrupted, and in some cases,
telegraphic offices were set on fire; auroras appeared both in the
Northern and Southern Hemisphere during the night which followed;
and the whole frame of the earth seemed to thrill responsively to
the disturbance which had affected the great central luminary of the
Solar System.
[Illustration: Fig. 32.—Copernican System: Facsimile of the Drawing
in the Volume by Copernicus Published in 1543]
The reader will now see why I have discussed relations which hitherto
he may perhaps have thought very little connected with my subject.
He sees that there is a bond of sympathy between our earth and the
sun; that no disturbance can affect the solar photosphere without
affecting our earth to a greater or less degree. But if our earth,
then also the other planets. Mercury and Venus, so much nearer the
sun than we are, surely respond even more swiftly and more distinctly
to the solar magnetic influences. But beyond our earth, and beyond
the orbit of moonless Mars, the magnetic impulses speed with the
velocity of light. The vast globe of Jupiter is thrilled from pole
to pole as the magnetic wave rolls in upon it; then Saturn feels
the shock, and then the vast distances beyond which lie Uranus and
Neptune are swept by the ever-lessening yet ever-widening disturbance
wave. Who shall say what outer planets it then seeks? or who, looking
back upon the course over which it has traveled, shall say that
planets alone have felt its effects? Meteoric and cometic systems
have been visited by the great magnetic wave, and upon the dispersed
members of the one and the subtle structure of the other effects even
more important may have been produced than those striking phenomena
which characterize the progress of the terrestrial or planetary
magnetic storms.
When we remember that what is true of a relatively great solar
disturbance, such as the one witnessed by Messrs. Carrington and
Hodgson, is true also (however different in degree) of the magnetic
influences which the sun is at every instant exerting, we see that a
new and most important bond of union exists between the members of
the solar family. The sun not only sways them by the vast attraction
of his gravity, not only illumines them, not only warms them, but he
pours forth on all his subtle yet powerful magnetic influences. A new
analogy between the members of the Solar System is thus introduced to
reinforce those other analogies which have been held so strikingly to
indicate that the ends for which our earth has been created are not
different from those which the Creator had in view when He planned
the other members of the Solar System.
The real end and aim of the telescope, as applied by the astronomer
to the examination of the celestial objects, is to gather together
the light which streams from each luminous point throughout
space. We may regard the space which surrounds us on every side
as an ocean without bounds or limits, an ocean across which there
are ever sweeping waves of light, either emitted directly from
the various bodies subsisting throughout space, or else reflected
from their surfaces. Other forms of waves also speed across those
limitless depths in all directions, but the light-waves are those
which at present concern us. Our earth is as a minute island placed
within the ocean of space, and to the shores of this tiny isle the
light-waves bear their message from the orbs which lie like other
isles amid the fathomless depths around us. With the telescope the
astronomer gathers together portions of light-waves which else
would have traveled in diverging directions. By thus intensifying
their action, he enables the eye to become cognizant of their true
nature. Precisely as the narrow channels around our shores cause the
tidal wave, which sweeps across the open ocean in almost insensible
undulations, to rise and fall through a wide range of variation, so
the telescope renders sensible the existence of light-waves which
would escape the notice of the unaided eye.
The telescope, then, is essentially a _light-gatherer_.
The spectroscope is used for another purpose. It might be called the
_light-sifter_. It is applied by the astronomer to analyze the light
which comes to him from beyond the ocean of space, and so to enable
him to learn the character of the orbs from which that light proceeds.
The principle of the instrument is simple, though the appliances by
which its full powers can alone be deduced are somewhat complicated.